1. Field of the Invention
The present invention relates to the preparation of esters by the carbonylation of monoolefinic compounds, namely, by reacting carbon monoxide and an alcohol with compounds containing a single olefinic double bond, and, more especially, relates to the preparation of diesters from alkyl pentenoates, e.g., the synthesis of alkyl adipates by the carbonylation of alkyl pentenoates.
2. Description of the Prior Art
It is well known to this art from Bulletin of the Chemical Society of Japan, Volume 46, pages 526 and 527 (1973), that a mixture containing alkyl diesters, and in particular an alkyl adipate, is obtained by reacting carbon monoxide and an alcohol with an alkyl pen-3-enoate under high pressure and at elevated temperature, in the presence of cobalt carbonyl and a heterocyclic aromatic nitrogen base. However, the industrial-scale development of such a technique, the value of which is not contested in principle, is greatly jeopardized not only by the poor efficiency of the catalyst system, but also by the considerable proportion of alkyl pentanoate formed, even though the reaction is carried out in the absence of hydrogen.
Furthermore, it too is well known to this art that the presence of small amounts of hydrogen in the reaction medium tends to increase the efficiency of cobalt-based catalysts in processes for the synthesis of esters by reacting an alcohol and carbon monoxide with an olefinic compound.
It has nevertheless been found that, in the majority of cases, this favorable effect associated with the presence of small amounts of hydrogen is accompanied by an adverse influence on the selectivity in respect of linear esters, which are the specifically intended products.
In fact, it has been observed that the presence of hydrogen not only tends to increase the proportion of hydrogenation products in the reaction mixture, but also is capable of reducing the proportion of linear ester in the esters formed.
This adverse effect greatly compromises the economics of these processes, insofar as the utilization of the branched esters and the hydrogenation product is uncertain or even nonexistent. This is the case, in particular, of the branched diesters and the alkyl pentanoates produced during the carbonylation of alkyl pentenoates. In fact, as these products are destroyed in practice, their formation in effect corresponds to an intolerable loss of starting material. Furthermore, hydrogen can be formed in situ from the traces of water which may be contained in technical-grade reactants, according to the well-known reaction: EQU H.sub.2 O+CO.fwdarw.CO.sub.2 +H.sub.2.
For obvious economic reasons, it would be desirable to be able to use technical-grade carbon monoxide containing hydrogen, without this being to the detriment of the selectivity in respect of linear esters, which are the desired final products. It would also be desirable, for the same reasons, to be able to use reactants containing traces of water, without this contributing to a loss in starting material.